The product obtained by the reduction of osmium tetraoxide in hydrochloric acid solution, followed by evaporation to dryness, has been shown to be p-oxo-hexachlorodiosmium(1V), Os2Ocl6. Infrared, magnetic, and mass spectrometric evidence are presented to substantiate this formulation. Salts of the p-oxo-decachloro-diosmate(1V) ion are readily isolated from hydrochloric acid solutions of the oxide chloride. Unsuccessful attempts were made to repeat the preparation of the compound said to be p-oxo-octachlorodiosmium(v). Reduction of osmium tetraoxide with thionyl chloride was shown to give osmium tetrachloride.